Over-clocking is down to the CPU and possible a little down to the PSU. RAM however makes no difference as to if you can OC your CPU or not.
Just to confirm, your 600w PSU is plenty to over-clock that E5400 with GTX 550Ti
Ram can very easily limit how much op can overclock. Op's rig isn't like your SB rig where you can set your ratios ect. 1:1 is the best for a 775 build but op will be limited there. Need to know what board and what ram he/she is using. That cpu might get 3.2ghz on stock cooling if everything is done right.